A telescopic sight is one type of aiming device that has long been used in conjunction with firearms such as rifles and handguns to assist a shooter in aiming the firearm. More recently, other aiming devices such as laser markers, range finders, and infrared imaging devices have been developed for use with firearms. These aiming devices are typically securely mounted to the firearm above a barrel or receiver of the firearm using a mounting rack or mounting rings. Some aiming devices, including telescopic sights, must be positioned at a particular distance from the eye of the shooter, known as the eye relief distance, to properly view the target through the aiming device. Once the aiming device is mounted to the firearm, fine adjustments are made to the mount, the aiming device, or both to accurately align the aiming device with the firearm. This fine tuning of the alignment is known as "sighting in." Because rifles are typically designed with mounting space sufficient for only one aiming device, the shooter is forced to choose one aiming device or to swap aiming devices in the field when needed. Each time aiming devices are swapped in the field, they must be sighted in to alleviate misalignment inherent in the mount's tolerances.
When used with a rifle, the telescopic sight is typically mounted at a fixed height above the barrel or receiver so that the shooter holding the rifle may easily view a target through the scope from the shooter's natural head position. Protective clothing and headgear, such as gas masks, bulletproof vests, and the like that are occasionally worn by the shooter, can change the shooter's natural head position and make it difficult to use the sight.
One conventional prior art device for mounting a telescopic sight to a rifle scope is shown and described in U.S. Pat. No. 5,035,487 issued to the assignee of the present invention. This mounting device includes a base unit securely mounted to the firearm in accurate alignment with the bore of the firearm and two split mounting rings for receiving a tubular housing of a rifle scope. The mounting rings each include a pin that extends from the rings below the telescopic sight for seating in one of a pair of receiving holes of the base unit for removably attaching the rings to the base unit. A locking lever locks the pins in the receiving holes. These devices do not accommodate the mounting of multiple aiming devices or other equipment to the firearm and do not allow any adjustment of the height of the telescopic sight above the firearm.
Another prior art type of mounting device is shown and described in U.S. Pat. Nos. D 269,989, D 269,988, and 4,328,624. This mounting device is known in the art as a WEAVER mount, after its original manufacturer, W. R. Weaver Company, El Paso, Tex. The WEAVER mount includes an elongate rail having a dovetail shaped cross section and multiple slots oriented transversely of the rail and spaced apart along the rail. Due to the WEAVER mount's widespread use in the industry and its ability to accommodate various types of aiming devices, it is considered a universal mount.
Yet another type of prior art mounting device having a dovetail rail is described in U.S. Military Standard No. MIL-STD-1913 (AR). This dovetail shaped mounting rail is known in the art as a Picatinny mount or Picatinny rail. As with the WEAVER mount, the Picatinny mount is a universal mount that provides a general purpose base for removably mounting different types of equipment to a firearm.
U.S. Pat. No. 4,707,772 describes a system for mounting a telescopic sight and a flashlight above a firearm. A pair of extension arms are substituted for a top portion of the split mounting rings of a conventional mount of the type shown in U.S. Pat. No. 5,035,487. A flashlight mount is provided at the distal end of the extension arms. This mounting system is suitable for mounting a flashlight above a rifle scope, but does not provide a general purpose mount for different types of auxiliary equipment, Furthermore, this mounting system lacks positioning features necessary to attain accurate alignment with the rifle of auxiliary equipment such as laser markers or infrared imaging devices. It would also not be suitable for mounting some types of auxiliary equipment because it would position the auxiliary equipment above the rifle sight at a height that would make it difficult for a shooter to view a target through the auxiliary equipment. This system is also prone to misalignment between the flashlight and the rifle scope, which can result from failure of the mounting rings to properly fit onto the rifle scope.
U.S. Pat. No. 5,134,798 describes a sight mount that comprises a two-tier base bar. The base bar provides a rigid support for two mounting rails, which may include rails of the Picatinny or WEAVER type. This device eliminates a number of the shortfalls of the mounting system of U.S. Pat. No. 4,707,772 described above. However, because of the rigidity required, the mount must be formed of metal having a thickness that significantly increases the weight of the rifle and decreases its portability. A vertical member of the mount that connects the two tiers of the base bar limits the allowable length of sights or equipment mountable to the lower tier. Finally, the different longitudinal positions of the two tiers make it difficult for a shooter to use two aiming devices having the same eye relief distance.